The overall objective of the proposed research is to use a new method to measure the acute and chronic effects of various drugs on learning in individual subjects and to study some possible interactions between the drug effects and situational variables. Pigeons will obtain food reinforcement via a four-response chain that requires pecking different keys in the presence of different colors. The sequence of correct key positions will change from session to session. Eventually, a steady state of repeated acquisition will be established for each subject, and this steady state will then serve as the learning baseline against which the effects of pre-session drug administration will be assessed. Dose-effect data (accuracy and pausing) will be obtained for several drugs, including methylphenidate, imipramine and magnesium pemoline, after which the effects of daily drug administration over long periods of time will be determined. The drugs will also be studied under comparable "performance" conditions, in which the subject is exposed to the same four-response chain each day. Finally, other variables in the situation that may interact with the drug effects (e.g., schedule of reinforcement) will be manipulated.